The most rapidly growing subgroup in the AIDS epidemic are young adults, with the majority of HIV transmission occurring through sexual activity during adolescence. More than half of all senior high school students report a history of sexual intercourse, with rates even higher among African-American and Hispanic youth. Only 58 percent of high school students reported condom use during their last sexual encounter. Therefore, reducing adolescents' risky sexual behavior is paramount to reducing overall HIV/ADS rates. Explanations of such adolescent risk behavior almost invariably refer to adolescents' beliefs in their invulnerability to harm. However, no study has used prospective, longitudinal methods with adolescents at risk for HIV to determine whether low risk judgments indeed motivate behavior or are instead reflective of behavioral experiences. As a New Investigator, I am committed to conducting research that informs and improves efforts to reduce adolescents' engagement in risky sexual behavior. Using a prospective cohort design in which ethnically and economically diverse male and female 9th graders are followed for four years, I propose to: (1) examine whether the onset of sex, broadly defined, is associated with previous and/or subsequent changes in judgments concerning STD and HIV/AIDS risk; (2) examine whether personal experiences with negative outcomes related to sex (or lack of such experiences) influence subsequent risk judgments and sexual behavior; (3) determine the extent to which perceived benefits of sex are related to adolescent sexual behavior, over and above perceived risks; (4) determine whether knowledge of peers' experiences with sex and related positive and negative outcomes plays a role in adolescents' risk judgments and subsequent sexual behavior; and (5) explore whether the relationship between risk judgments and sex varies by gender, race, economic status, or development. Approximately 600 ninth graders recruited from schools in San Francisco will be surveyed in class every 6 months, for a total of seven data collection periods. Data to be collected at each data wave include: perceptions of risk to STDs and HIV/AIDS, perceived benefits of sex, current and past sexual behavior, intentions and motivations to have sex, personal experiences with behavior-linked negative outcomes, knowledge of peers' sexual activity and experiences with positive and negative outcomes, and cognitive and psychosocial development. The results of this research should provide information concerning the relationship between risk judgments and behavior that is expected to be valuable to researchers and health practitioners concerned with developing programs to reduce adolescents' HIV risk.